This invention relates to a photosensitive composition and a photosensitive lithographic printing plate, more particularly to a photosensitive composition suitable for a positive-type photosensitive lithographic printing plate and a photosensitive lithographic printing plate by use thereof.
A positive-type lithographic printing plate has an ink-receiving photosensitive layer on a hydrophilic support, and an image exposure is effected on the photosensitive layer, followed by developing, thereby forming an image by removing the non-image portion while permitting the image portion to remain. In the case of practical printing, the lipophilic property of the image portion and the hydrophilic property of the non-image portion are utilized.
Ordinarily, in the photosensitive layer of the positive-type lithographic printing plate, an o-quinonediazide compound as the photosensitive component, and an alkali soluble resin as the component for enhancing the film strength and the alkali solubility are contained.
In carrying out the so-called multi-face printing in which a plurality of film originals are sequentially printed with varied positions on a photosensitive printing plate by use of such composition, it has been desired to shorten the printing time, namely sensitivity elevation, from the standpoint of working.
The photosensitive layer by use of an o-quinonediazide compound, when handled under white fluorescent lamp, suffers from light fogging, and when subjected subsequently to developing, the photosensitive layer at the image portion is invaded at the image portion to cause reduction of film, whereby the trouble of lowering in printing power has occurred frequently. For this reason, it has been desired to have a photosensitive lithographic printing plate having a photosensitive layer improved in resistance to light fogging by white fluorescent lamp (hereinafter called "safe light characteristic").
Further, developing processing of the photosensitive lithographic printing plate by use of an o-quinonediazide compound is performed generally in a developer of an aqueous alkali solution, but the developing ability of the developer is susceptible to fluctuation by various conditions, whereby the developing ability may be sometimes lowered by fatigue by treatment of a large amount or deterioration by air oxidation and the photosensitive layer at the non-image portion of the printing plate may not be completely dissolved even when processed. On the contrary, due to elevation of the bath temperature accompanied with excessive supplemented amount of the developer or elevation of the air temperature, the developing ability may sometimes become higher than that defined, whereby the image portion of the printing plate may be invaded or the dot may disappear. Particularly, when the image line portion at the portion where a worker has touched with a finger during multi-face printing working, etc., there has been involved the problem that it was invaded in the shape of a fingerprint.
Also, the dot reproducibility accompanied with fluctuation in the processing conditions such as various fluctuations of the developer, etc. as mentioned above has been a problem. The fluctuation in dot reproducibility is a crucial problem because it is revealed as fluctuation in gradation. Particularly, when reproducibility of small dot is inferior, the reproducibility at the high light portion in the printed product obtained has posed a problem.
Further, a positive-type photosensitive lithographic printing plate has also the drawbacks that it is weakly resistant to various processing chemicals used during printing, for example, isopropyl alcohol, UV ink, plate cleaner, etc. contained in wetting water, and consequently lowered in printing resistance.
For this reason, the photosensitive lithographic printing plate has been desired to have broad developing tolerance which exhibits developing characteristics similar to the standard developer even in a developer lowered in processing ability or a developer with the processing ability potentiated higher than the standard. (In the following, the tolerable range of lowered developing ability to give adequate developing result is called underdevelopability, and the tolerable range of elevated developing ability to give adequate developing result is called overdevelopability).
As a means for solving the above problems, for example, Japanese Provisional Patent Publication No. 121044/1984 discloses addition of an amphoteric surfactant and an organic boron type surfactant for improvement of sensitivity and developing tolerance, but it had no effect in improvement of the above problem.
Further, Japanese Provisional Patent Publication No. 251740/1987 and U.S. Pat. No. 3,868,254 disclose improvement of developing tolerance by addition of a nonionic surfactant, but it was still insufficient concerning sensitivity and safe light characteristic. Also, it had the drawback that foaming of the developer occurred when performing developing processing.
Accordingly, it has been desired to have a photosensitive composition suitable for photosensitive lithographic printing plate having good sensitivity and further little foaming of the developer without damaging safe light characteristic, developing tolerance, and a photosensitive lithographic printing plate.